The research program is directed towards appraising the determinants of mental development from birth to 8 years of age. The data are drawn from 300 twin pairs (plus their siblings) who were recruited at birth and have been tested periodically throughout childhood. Genetic factors are expected to make a significant contribution to mental development at all ages, but their influence may be modified by prenatal and birth effects, and by the comparative advantages or disadvantages afforded by the home environment. The proposed research will deal directly with the differential influence of genetic factors by examining the course of mental development throughout childhood for advantaged vs. disadvantaged twins. The aim is to detect the ages at which developmental status begins to lag for disadvantaged twins, and to identify the capabilities that are most affected. The results can be incorporated into a remedial program, and the twin sample furnishes an ideal matched control group for testing the effectiveness of the program. The project also focuses on the contribution of genetic factors to school-age IQ as this may be manifested in the overall IQ score, in the pattern of subtest scores, and in the consistency of the subtest pattern over ages. The results will disclose whether some components of intellectual functioning are more heavily influenced by genetic factors, while others are more susceptible to home- and school- environmental influences.